For the production of single-component molded plastic parts, injection molding and compression processes (injection compression molding) can be used. Typical injection molding processes without a compression technique are used for the production of relatively small injection molded parts, in the case of which short flow paths occur and it is possible to work with moderate injection pressures. In an injection molding process, generally, a polymer melt is injected into a cavity formed between two closed, positionally fixed mold plates and solidifies there.
Injection compression molding processes differ from typical injection molding processes in that the injecting and/or solidifying operation is carried out while performing a mold plate movement. The polymer melt is compressed while performing mold movement in the closing direction. In particular, in the production of large-area and thin walled molded parts, the compression technique can be suitable, since in this way, a lowering of the high injection pressures required in the case of large molded parts can be achieved. Furthermore, stresses or warpage in the injection molded part, which generally occur as a result of high injection pressures, can be avoided.
The film gate process can be used to avoid sprue marks on sheet or shaped plastic parts. For optical parts, sprue marks cannot be tolerated. For the film gate technique, the injection molding of the shaped part takes place laterally by means of a film plate. The film plate has a runner, which opens into the component cavity at the edge. After the material has cooled down and the shaped part has been removed from the injection mold, it is provided with the film sprue, of a form corresponding to the runner in the film plate. After removal, the film sprue must be detached from the shaped part. On account of the lateral or edge side injection molding performed in the film-gate technique, no sprue marks occur on the main surfaces of the shaped part. However, an additional working step is required for sprue detachment, which can cause loss of material, soiling of the shaped part, and optical defects at the point of detachment can be an issue.
Injection compression molded glazing parts often require maximum optical clarity. Therefore, during injection molding, the lowest possible stress levels should be reached. Injection compression molding further requires no backflow of the material from cavity to the runner, which can result in undesirably high shrinkage in the gate region of the shaped part. In a typical shut off nozzle system, the shut off pin has an influence on the stress and the optical quality of the part produced. Thus, there is a need for a system that can prevent backflow of the material into the runner alternatively, or in combination with a free flowing nozzle without a pin in the flow of the material.